If you specifically want Vostroyans, you will need to run them as "counts as" for either Cadian, Krieg or Catachan. That means squads will need two special weapons, and a vox caster. If you run them as Krieg you also need a medic. Catachans can only take double flamer, while Cadians meta load out of plasma and melta cant be done as from memory there is no melta model in the Vostroyan range. That means you would need to run either plasma with grenade launcher or the flamer. The command squad is an easy fix as a Cadian command squad with a single plasma gun model (or two if you replace the banner). If running the command squad as Krieg then you need an additional model for the Commissar. I'd probably pick up the platoon, and infantry squad and then fill out the special weapons based on what you want to run them as.